With quick code analysis, it seems that the CapeFeatureRenderer uses abstractCleintPlayerEntity.isInSneakingPose() rather than the model's sneaking field.

Likely a fairly simple redirect.

Have yet tested with an elytra.

Evidently, that was for the angle of the cape, not the Y positioning; although it does make the cape less visually awkward.

Come to realise it; the LadderAnimation does not seem to be properly applying a pose overwrite on 1.21, although it seems capable of overwriting the model's sneaking overwrite fine.

Will have to look into it. Tbh had 0 time so far to play 1.21 myself.

Hm, this has been fixed in 1.21.3+, but is broken for everything else 🤔. So probably broke during some refactoring.

Edit: The mixin is straight up not hooked up for anything other than 1.21.2+. Amazing.

Ok I think I won't open this can of worms right now, 1.21.3+ has it fixed from there on. But turns out the head layers don't play nice with the head limiting, so that's something to fix before releasing the update.